In recent years, optical surgeries by which the shapes of corneas are altered with laser are attempted. At the time of such a surgery, it is important to precisely measure the aberrations of the eye. Before and after a corneal surgery or a cataract surgery, visual function tests, including measurement of visual acuity, a low-contrast visual acuity test (which is also referred to as “contrast visual acuity test”) and a contrast sensitivity test, are performed.
Here, a general visual acuity test using a standard visual acuity chart measures the visual acuity to a minute object with a high contrast ratio. Then, in a cataract patient, for example, turbidity of a crystalline lens causes scatter of light, which lowers the contrast of an image on the fundus but does not affect deviation of the optical focus position of the image. As a result, the cataract patient as a subject may be able to read the image as a letter although the image on the fundus is blur. Also, in the case where a spherical aberration is caused by optical distortion of a cornea and a crystalline lens, the subject may be able to read the image as a letter although the image on the fundus is blur.
Contrast sensitivity which represents the ability to detect a specific object with a minimum contrast is an important index to express the visual ability of a subject in daily and social life. Thus, in measurement of contrast sensitivity, a sine wave grid pattern is used to examine a single modulation transfer function. A sine wave grid pattern is a grid pattern in which the density varies in a sine wave pattern. In a contrast sensitivity test, the minimum contrast necessary to recognize a grid pattern is measured at various finenesses (spatial frequencies).
However, since a contrast sensitivity test is performed in such a manner that a subject (patient) reads a visual acuity chart under examiner's direction, there are the following problems.    (1) As the test is performed using a single visual acuity chart, the subject remembers the visual acuity chart when trials are repeated. For example, when the left eye is examined after the right eye and when the visual acuity of the right eye is better than that of the left eye, the subject can respond correctly with memory even if it cannot see the chart with the left eye. Thus, it is difficult to measure the contrast sensitivity of a subject precisely.    (2) It takes about 10 minutes to measure contrast sensitivity and a clinical technologist or ophthalmologist must concentrate on the test during that time. Thus, the practice time or the examination time is unavoidably long.    (3) The diameter or area of the pupil region of a subject is preferably constant during measurement of contrast sensitivity. However, as the eye gets tired, the diameter or area of the pupil region of the subject varies and becomes different from the theoretical diameter or area of the pupil region of the subject. Thus, it is difficult to measure the contrast sensitivity of a subject precisely.